There is known a cold forming tap which has an external thread portion including radially outwardly protruding portions and relief portions which are adjacent to and have a diameter smaller than the protruding portions. The protruding portions and the relief portions are alternately located as viewed in a helical direction of the cold forming tap in which a thread extends. The protruding portions are substantially equi-angularly spaced apart from each other as viewed in a circumferential direction of the cold forming tap, while the relief portions are also substantially equi-angularly spaced apart from each other as viewed in the circumferential direction. For forming an internal thread in a prepared hole formed in a workpiece, the protruding portions are forced into a surface of the prepared hole, for plastically deforming the surface so as to form the internal thread on the surface. As one type of the cold forming tap, there is proposed a cold forming tap having an internal finish cutting edge which is provided to finish a minor diameter of the formed internal thread.
This cold forming tap having the internal finish cutting edge is advantageously capable of eliminating a shape of incomplete thread ridge which may possibly appear on a crest of the internal thread, i.e., on the minor diameter of the internal thread. The cold forming tap having the internal finish cutting edge has another advantage of preventing a breakage of the tap due to an excessive torgue acting on the tap, which is caused by a large amount of surplus stock of the workpiece plastically flowing into a limited space, in a case where an internal thread is to be formed in a hole previously formed through the workpiece, which hole has a dimension smaller than a lower limit, such as a tapered hole formed in the workpiece in the casting process. Further, the cold forming tap having the internal finish cutting edge advantageously assures an improved accuracy in the minor diameter of the internal threads.
In the above-described conventional cold forming tap having the internal finish cutting edge, the highest portions of a root diameter of the external thread portion are located at the relief portions adjacent to the protruding portions (lobes) which correspond to the highest portions of a major diameter of the external thread portions, and flutes are formed at the highest portions of the root diameter so that the highest portions of the root diameter constitute the cutting edges.
In general, the external thread portion of the cold forming tap is formed, in a step referred to as a thread grinding, by a grinding wheel having a thread profile formed on an outer circumferential surface thereof such that the thread ridge and the root of the external thread portion are simultaneously formed. In the above-described conventional cold forming tap having the internal finish cutting edge, the crest and the root of the external thread portion have respective concave-convex shapes in the cross section perpendicular to an axial direction of the tap. However, the respective concave-convex shapes of the crest and the crest and a concave-convex shape defined by the root do not coincide with each other as viewed in the circumferential direction of the tap, resulting in a need of grinding the ridge and the root separately from each other. The separate grinding operations not only complicate the production process, but also make it difficult to control the dimensions of the thread ridge. For obtaining high dimensional accuracy of the thread ridge, it would be indispensable to improve the accuracy of the entirety of the external thread portion. Consequently, the conventional cold forming tap has a drawback of requiring a high level of grinding technique and a high cost for the production. The present invention has an object of providing a cold forming tap having an internal finish cutting edge and a method of producing the cold forming tap, wherein the external thread portion can be formed basically in a simple process as known in the art, with easy dimensional control of the thread ridge, at a reduced cost.